Airframe manufacturers have found that skins of graphite, boron, or a blend of graphite and boron composite (collectively “polymeric composite”) unidirectional tape can be strengthened by the introduction of titanium foil between laid up plies of polymeric composite unidirectional tape. For example, U.S. Pat. No. 5,866,272 to Westre et al., incorporated by this reference, is one of several patents teaching the placement of titanium foil between plies of polymeric composite unidirectional tape.
The resulting skin including a titanium foil makes up for a bearing deficiency of non-titanium containing polymeric composite panels, which ordinarily will not bear loads or hold pins well. In addition, composite construction is typically one-third of the density of titanium. Titanium has a relatively low fatigue strength and relatively high crack growth rates. Introducing titanium foil to the polymeric composite skins yields a resulting hybrid that exploits the strengths of polymeric composite and titanium construction. With the introduction of sufficient titanium foil, the resulting skin approaches and can surpass the strength properties of either material alone.
Traditionally, adding plies of polymeric composition has made pad-ups for joints and higher-loaded areas of composite panels. The added plies are laid up in balanced and symmetric manner to form a ramp at the site of the pad-up. These pad-ups add strength at points in the skin without the addition of weight by uniform addition of material across the skin. Recently, the virtues of introduction of titanium foil have been achieved by padding up by adding titanium foil plies.
The selective introduction of titanium foil at places of greater pin bearing loads causes a resulting skin of non-uniform thickness. The transition in thickness from polymeric composite skin alone to reinforced hybrid composite skin is known as a ramp. Fastening parts to pad-ups typically requires complex machining and close tolerances to conform to the ramp. Such considerations mean greater expense in manufacturing.
Additionally, as the thickness of the composite skin increases, the bolt bending stresses increase, consequently decreasing the strength of skin at the bolthole. For this reason, minimizing of the pad-up is advantageous. Thus, there is an unmet need in the art for a method of introducing titanium foil to a polymeric composite lay-up in a manner to reduce the pad-up and ramp length.